The circuit designer now has two ways to specify a single layer capacitor.
The first way is by 1MHz cap and equivalent series inductance. This is
the circuit theory point of view. The second way is by characteristic
impendence and electrical length. This is the transmission line theory
point of view. It is the circuit designer's responsibility to decide the
more effective way for a particular application. Guidelines for capacitor
choice are presented below, using some examples from CAPCAD.
For the purpose of these exercises, low frequency shall be defined as
fmax < fp/2 where fmax is the maximum operating frequency, and fp is
the first parallel resonance of the capacitor.
Example one: Low frequency turning capacitor
D25CG2ROC5PX(1=.025,W=.025,t=.0045,K=65,df=.007,R=.03) has its first
parallel resonance at '32.6 GHz. At any frequency below 16 GHz the
physical capacitor may be represented by an ideal 2.0 pF capacitor with a
series 0.012 nH inductor. Losses may be approximated by choosing a function
for Q, such as:
Q(f)=120x(f(GHz))-.176 1 Ghz < f < 10 Ghz
or by defining series and parallel resistors, such as:
Rser=.003 x (f/1 GHz).5
1 GHz < f,10 Ghz
Rpar= 120/(2 fC)
The equivalent series inductance of the manufactured device will vary by +- .002 nH.
Once the secondary parameters of series inductance and loss have been specified,
a capacitance value can be chosen for the application. To ensure the circuit will
work as designed, it is a good idea to write CAPCAD'S parameters to a file and use
them in the final circuit simulation.
When the modeled equivalent series inductance of a capacitor is low enough that it
doesn't appreciably affect the device's reactance, the circuit theory point of view
is adequate. Specify the capacitor by its case size and capacitance value.
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